We have identified the IFITM family of transmembrane proteins as important regulators of endothelial cell (EC) sprouting in vitro, and angiogenesis in vivo. During angiogenesis EC shift from quiescent cells in the walls of mature vessels, to migrating and proliferating cells as part of a sprout, to cells capable of generating lumens and forming anastomoses, and back to quiescent cells as the new vessel stabilizes. In the previous round of funding we identified a role for the notch signaling pathway in suppressing proliferation and branching at the tips of new capillary sprouts, work that has now been corroborated in several different labs. In this competing renewal we present data showing that in human the IFITM family of proteins are constitutively expressed in the vasculature and that they are expressed reciprocally in developing sprouts - IFITM1 in tip cells and IFITM2 in trunk cells. Both misexpression and knockdown of IFITMs disrupts sprouting and lumen formation in vitro and angiogenesis in vivo, and both block proliferation by inducing p21 expression. IFITM2, but not IFITM1, synergizes with notch signaling. The hypothesis we will test in this competing renewal is that the regulated expression by EC of the IFITM family of proteins is critical to the proper formation of lumenized vascular sprouts. We will use a combination of in vitro and in vivo approaches to test this hypothesis and to further our understanding of the role of this family in angiogenesis. Specifically, we will address the following three aims: 1. Determine the role of the IFITMs in endothelial sprouting and tube formation 2. Determine the mechanism(s) of IFITM signaling in endothelial cells 3. Identify the critical domains of IFITM1 and IFITM2 and their interacting partners